1. Field of the Invention
The present invention relates, in general, to positive-displacement oil pumps and, more particularly, to a positive-displacement oil pump designed to reliably feed a sufficient amount of lubricating oil to a displacement compressor of refrigeration systems, such as refrigerators or air conditioners, regardless of a variation in the operational speed of the compressor.
2. Description of the Prior Art
As well known to those skilled in the art, conventional compressors for refrigeration systems typically use a centrifugal oil pump which is housed in a crankshaft and feeds lubricating oil to moving parts of a compressor using centrifugal force generated by rotation of the crankshaft.
A conventional centrifugal oil pump for compressors of refrigeration systems will be described herein below with reference to FIGS. 1 and 2.
FIG. 1 is a view of a crankshaft 150 of a refrigerant compressor, which includes a conventional centrifugal oil pump 100. FIG. 2 is a view showing an oil cap 110 and a propeller 120 included in the conventional centrifugal oil pump 100.
As shown in the drawings, the crankshaft 150 of a refrigerant compressor has a shaft body 151, with the conventional centrifugal oil pump 100 provided in the lower portion of the shaft body 151 such that the lower end of the pump 100 is immersed in lubricating oil contained in an oil reservoir 130.
That is, a central opening 152 is axially formed in the shaft body 151 of the crankshaft 150, and axially receives the centrifugal oil pump 100. An oil-feeding hole 153 is formed in the shaft body 151 such that the oil-feeding hole 153 extends from the top end of the central opening 152 to the outer circumferential surface of the shaft body 151. An oil guide groove 154 is formed around the circumferential surface of the shaft body 151 such that the oil guide groove 154 extends from the outside end of the oil-feeding hole 153 to a crank pin 155 provided at the upper end of the shaft body 151.
The conventional centrifugal oil pump 100 comprises an oil cap 110 and a conical propeller 120. The oil cap 110 consists of a cylindrical body part 111 and a conical tip part 112, with the conical propeller 120 axially set in the conical tip part 112.
When the crankshaft 150 is rotated, the centrifugal oil pump 100, axially inserted in the central opening 152 of the crankshaft 150, is also rotated. During such a rotation of the oil pump 100, lubricating oil contained in the oil reservoir 130 is introduced into the oil cap 110 through an inlet of the oil cap 110, and is forcibly lifted upward to the oil-feeding hole 153 due to centrifugal force generated by the rotation of both the propeller 120 and the central opening 152. At the outside end of the oil-feeding hole 153, the oil further flows forcibly upward through the oil guide groove 154 due to rotation of the crankshaft 150 relative to a journal bearing, thus reaching the crank pin 155 prior to being sprayed into the interior of a compressor's shell. The frictional contact surfaces of moving parts in the shell are thus lubricated.
In other words, when the crankshaft 150 is rotated, the centrifugal oil pump 100, provided in the lower portion of the crankshaft 150, is also rotated. During the rotation of the oil pump 100, the propeller 120 and the central opening 152 generate a pumping force for upwardly pumping the lubricating oil to a predetermined pumping head.
However, the oil pumping function of the crankshaft 150, which includes the centrifugal oil pump 100 and the central opening 152, is only due to the centrifugal force generated by rotation of the crankshaft 150. Therefore, when the rotating speed of the crankshaft 150 falls below a predetermined reference level, the pumping head of the oil pump 100 is quickly reduced. This means that it is almost impossible to feed an effective amount of lubricating oil to the moving parts inside the compressor's shell when the crankshaft 150 is rotated at low speed. In such a case, the moving parts of the compressor may suffer excessive abrasion at their frictional contact surfaces, and, furthermore, the heated frictional contact surfaces of the moving parts or the heated motor of the compressor may not be effectively cooled, thus occasionally causing overheating and severe damage. This results in severe damage or breakage of the compressor.